[libvirt] Problems with "attach-interface" command of libvirt

Dear libvirt-developer: I'm a student of computer science and technology in China. I encountered a problem when I want to attach a bridge to an existed Virtual Machine. I used the command "attach-interface domain_id bridge br0 --target tap0". "br0" was created by brctl and "tap0" was created by tunctl.Here is the error information: error: Failed to attach interfaceerror: internal error unable to send TAP file handle: No file descriptor supplied via SCM_RIGHTS.

I know there are some bugs in the earlier version of libvirt. So I upgraded the ubuntu kernel to 2.6.38.8 and added the module "macvtap" by the using the command "sudo modprobe macvtap". I have installed libvirt0.8.8, QEMU 0.8.8 and QEMU 0.12.3.Although I did all the above, the problem is still not solved. I need your help sincer!
ely. Tao Siheng &nbsp!
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sp; 2011.9.1